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The Moon's dark cone-shaped umbral shadow first touches down on the Earth's surface at sunrise in northern Canada. The long but narrow path of the total solar eclipse (about 150 miles wide) starts in Canada, then swings eastward across the Arctic, Russia, Mongolia and China. The Moon's shadow takes only about 2 hours to make the trip from northern Canada to eastern China, where the total eclipse ends at sunset. Thereafter, the Moon's umbra lifts off the Earth's surface and falls into the vastness of space. Here's a map of this eclipse, courtesy of Fred Espenak's Eclipse Web Site
When the total solar eclipse is seen in the early morning (after sunrise) or the late afternoon (before sunset) - near the start and the end of the eclipse path - totality lasts for about 1 minute and 40 seconds at any one locality. Localities at the middle part of the eclipse path see the total eclipse lasting nearly 2.5 minutes.
Explanation: Angle of Moon's Shadow
The Earth's shadow is tangent to the Earth's surface at the sunrise and sunset points on the eclipse path. And at early morning and late afternoon, the Moon's shadow falls on the Earth's surface at a very oblique angle. Therefore, the Moon's shadow races most quickly across the Earth's surface near the start and the end of the eclipse path. Around the middle of the eclipse path, the Moon's shadow reaches a minimal speed (relative to the Earth's surface), because it hits the Earth more head-on. This animation helps to illustrate.
Moon Closest Around Mid-Eclipse
The Moon is closest and largest to the eye at localities near the center of the eclipse path. Points on the Earth's surface that reside at the start or the end of the eclipse path loom farther from the Moon.
Hence, as seen from Earth, the Moon's angular diameter is smaller at the beginning and the end of the eclipse path, but larger in the mid-section. Because of this, there are times when the varying distance between the Earth's surface and the Moon can give rise to what are called hybrid (or annular-total) eclipses. See the footnote below.
Why Is Totality Not Longer for the 2008 August 1 Eclipse?
The longest possible duration for a total solar eclipse is about 7.5 minutes. For a long-lasting eclipse to occur, several things are necessary.
Aphelion & Perigee
First, the Earth must be appreciably close to aphelion - at the spot in its orbit when the Earth is farthest from the Sun for the year. Yearly, the Earth reaches aphelion in early July. We're about 3 million miles farther away from the Sun at aphelion than we are at perihelion - our closest point to the Sun, which occurs annually in early January.
Second, the Moon must be appreciably close to perigee - the Moon's closest approach to the Earth for the month. At perigee, the Moon is approximately 30,000 miles closer to Earth than when it's at apogee - the Moon's most distant point in its orbit.
Since the Sun was at aphelion on July 4 and the Moon was at perigee on July 29, it seems like the maximum duration of the August 1 total solar eclipse should exceed 2.5 minutes. We'll investigate the reason for the relative shortness of totality, even though this eclipse took place in fairly close conjunction with aphelion and perigee.
Longest Total Solar Eclipse in the 21st Century
The next total solar eclipse will fall on July 22, 2009, with the maximum totality lasting for 6 minutes and 39 seconds. This will be the longest total solar eclipse in the 21st century. Yes, the 2009 total solar eclipse will happen a bit closer to aphelion than the 2008 eclipse. And with perigee falling on July 21, 2009, the 2009 total eclipse will fall a little closer to perigee. Even so, it doesn't seem like the duration of totality between these two eclipses should be so divergent. So what gives?
Longest-lasting Total Solar Eclipses Happen Near the Equator
For a long-lasting eclipse to take place, the Earth has to be appreciably close to aphelion and the Moon has to be appreciably close to perigee. But that's not all! The total solar eclipse path has to pass through the tropics. Contrast the paths of the August 2008 and the July 2009 total solar eclipses. The 2008 total solar eclipse path crosses the far northern latitudes, whereas the 2009 total solar eclipse path dips into the tropics.
Moon's Shadow Moves Most Slowly at Equator
The Moon's shadow moves in the same direction that the Earth rotates: from west to east. Even though the Moon's shadow moves more swiftly than the Earth rotates, a point on the Earth's surface rotates faster near the equator, and more slowly near the poles. The Earth rotates at over 1000 miles per hour at the equator, but at only half that speed at 60 degrees north and south latitude.
Since the Earth rotates most quickly near the equator and most slowly near the poles, that means the Moon's shadow moves most slowly over the tropical regions and most quickly near the poles. Hence, with all other factors being equal, we can expect a longer total solar eclipse when the Moon's shadow sweeps close to the equator.
Moon & Sun at Zenith
One last thing: the closer the total solar eclipse is to the zenith (straight overhead), the larger the Moon's angular diameter and the longer the Moon covers over the Sun. Notice where the greatest eclipse and sub solar points are for the August 1, 2008 and July 22, 2009 total solar eclipses. Sub solar refers to the point on the globe where the Sun is at zenith at the moment that the greatest eclipse is taking place. The nearer the sub solar and greatest eclipse points are to one another, the closer the greatest eclipse is to the observer's zenith.
Maximum Duration For Centuries To Come
For an especially long-lasting total solar eclipse to occur, the Earth must be close to aphelion, the Moon must be close to perigee, and the greatest eclipse shadow must fall near the equator. The longest total solar eclipse for a number of centuries to come will occur on July 16, 2186, exhibiting a maximum duration of 7 minutes and 29 seconds!
copyright 2008 by Bruce McClure
|Live Webcast of the 2008 August 1 Total Solar Eclipse!|
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Footnote: April 8, 2005 Solar Eclipse
Some solar eclipses are hybrid (annular-total) eclipses. When the Moon is close enough to Earth to completely cover over the Sun's disk, it's a total solar eclipse. When the Moon's too far to totally cover over the Sun's disk, it's called an annular eclipse, with an annulus - or thin ring - of sunshine encircling the New Moon silhouette.
During the eclipse of April 8, 2005, it was an annular eclipse at the beginning and the end of the eclipse path but a total solar eclipse over the central portion. That's because the Moon is farther away from the Earth's surface when the eclipse is viewed around sunrise and sunset, but closer to the Earth's surface when the eclipse is seen closer to noon. The eclipse map shows the annular solar eclipse as a red line and the total solar eclipse path in blue.
July 2008 Feature * Sept 2008 Feature